The goal of this study was to examine whether hyperactivity is a uniformly pervasive symptom of ADHD or whether it is especially evident when children with ADHD are required to perform tasks that tax their working memory (WM) capacity. The researchers predicted the latter based on the premise that increased motor activity can enhance the cortical arousal required during tasks that place high demands on WM, and that children with ADHD have significant WM deficits relative to peers.

Participants were 23 boys aged 8 to 12 years; 12 were diagnosed with ADHD, combined type and 11 did not have any psychiatric disorder. All boys engaged in a series of computer tasks that varied in the degree to which successful completion depended on WM. In the low WM condition, boys played with a program which allowed them to draw/paint anything they wanted using a variety of interactive tools. In the WM conditions, they were required to correctly recall either a sequence of numbers/letters in reverse order (verbal WM) or the locations of a series of dots that were flashed on the screen (visuo-spatial WM). Different numbers of number/letters and dots were used on different trials so that children’s maximum WM capacity in each domain could be measured, i.e., the larger the sequence they could recall the greater their capacity.

Boys wore actigraphs on both ankles and one wrist during the computer tasks. An actigraph is an acceleration-sensitive device that is worn like a watch and that provides a precise measure of motor activity. This enabled the researchers to obtain highly accurate measurements of boys’ activity during the computer tasks that varied significantly in their WM demands.

Results indicated that during all computer tasks – even those that imposed low WM demands – boys with ADHD demonstrated significantly higher movement rates than control boys. However, differences in activity level were relatively small during the low WM task and of considerably greater magnitude during tasks that depended heavily on WM – and when researchers conducted the analyses controlling for WM differences between the groups of boys, differences in activity level were no longer evident.

The researchers interpreted their results as suggesting that all children are prone to show greater motor activity when engaged in tasks that place a high demand on WM. They hypothesize that this occurs because motor movement stimulates cortical activity that helps all children with successful performance of such tasks. However, because children with ADHD have significantly lower WM capacity than peers, the impact of highly demanding WM tasks on their motor activity is greater. They suggest that this finding helps explain “…anecdotal parent and teacher reports that children with ADHD remain engaged in particular tasks and activities with no apparent excessive motor activity (e.g., video games, watching TV), yet move excessively during most in-seat academic/learning activities. In particular, their findings suggest that excessive motor activity will vary as a function of the WM demands of the task that children are involved in, which is why excessive activity is more evident in academic-related tasks that frequently depend on WM.